Sex and the frontal cortex: A developmental CT study in the spotted hyena

The purpose of this study was to examine developmental and individual variation in total endocranial volume and regional brain volumes, including the anterior cerebrum, posterior cerebrum and cerebellum/brain stem, in the spotted hyena (Crocuta crocuta). The spotted hyena is a highly gregarious animal noted for living in large, hierarchically organized groups. The social lives of male and female spotted hyenas do not differ until after puberty, when males disperse from the natal group, while females remain philopatric. Here we sought to determine whether the divergent life histories of male and female spotted hyenas are linked to differences in brain size or organization. Three-dimensional virtual endocasts were created using computed tomography from 46 spotted hyenas skulls (23 females, 22 males, 1 unknown sex) ranging in age from 1 day to 18 years. Brain volume and skull length were highly correlated (r = 0.91), and both reached asymptotic values by 34 months of age. Analyses of total endocranial volume (relative to skull length) and cerebellum/brain stem volume (relative to total endocranial volume) revealed no sex differences. However, relative anterior cerebrum volume, comprised mainly of frontal cortex, was significantly greater in adult males than adult females, and relative posterior cerebrum volume was greater in adult females than adult males. We hypothesize that the demands of neural processing underlying enhanced social cognition required for successful male transfer between matriarchical social groups at dispersal may be greater than cognitive demands on philopatric females.